Nitrate‐dependent iron(II) oxidation in paddy soil

Abstract

Iron(III) profiles of flooded paddy soil incubated in the greenhouse indicated oxidation of iron(II) in the upper 6 mm soil layer. Measurement of oxygen with a Clark-type microelectrode showed that oxygen was only responsible for the oxidation of iron(II) in the upper 3 mm. In the soil beneath, nitrate could be used as electron acceptor instead of oxygen for the oxidation of the iron(II). Nitrate was still available 3 mm below the soil surface, and denitrifying activity was indicated by higher concentrations of nitrite between 3 and 6 mm soil depth. Nitrate was generated by nitrification from ammonium. Ammonium concentrations increased beneath 6 mm soil depth, indicating ammonium release and diffusion from deeper soil layers. High concentrations of ammonium were also found at the surface, probably resulting from N2 fixation by cyanobacteria. Experimental adjustment of the nitrate concentration in the flooding water to 200 microM stimulated nitrate-dependent iron(II) oxidation, which was indicated by significantly lower iron(II) concentrations in soil layers in which nitrate-dependent iron(II) oxidation was proposed. Soil incubated in the dark showed high iron(III) concentrations only in the layer where oxygen was still available. In this soil, the nitrogen pool was depleted because of the lack of N2 fixation by cyanobacteria. In contrast, soil incubated in the dark with 500 microM nitrate in the flooding water showed significantly higher iron(II) and significantly lower iron(II) concentrations in the anoxic soil layers, indicating nitrate-dependent iron(II) oxidation. Anoxic incubations of soil with nitrate in the flooding water also showed high concentrations of iron(II) and low concentrations of iron(II) in the upper 3 mm. As oxygen was excluded in anoxic incubations, the high iron(III) concentrations are a sign of the activity of nitrate-dependent iron(II) oxidizers. The presence of these bacteria in non-amended soil was also indicated by the most probable number (MPN) counts of nitrate-dependent iron(II) oxidizers in the layer of 3-4 mm soil depth, which revealed 1.6 x 10(6) bacteria g(-1) dry weight.

Anaerobic Nitrate-Dependent Iron(II) Bio-Oxidation by a Novel Lithoautotrophic Betaproteobacterium, Strain 2002

“…However, vertical geochemical profiles of Campus Lake sediment contained coexisting nitrate and dilute HCl-extractable Fe(II) concentrations. The coexistence of ambient nitrate and Fe(II) concentrations is not restricted to Campus Lake sediments, as similar geochemical profiles have been observed in rice paddy soil cores (40). The aforementioned thermodynamic predictions do not account for dynamic microbial metabolic processes generating nitrate.…”

Phylogenetic and Functional Diversity of Denitrifying Bacteria Isolated from Various Rice Paddy and Rice-Soybean Rotation Fields

“…Until this report, only the above two Pseudogulbenkiania strains had been reported (16,28); however, our results suggest that these bacteria may be ubiquitous in rice paddy soils. Since reduced metals and fertilizer-N are available in paddy soils, the Pseudogulbenkiania strains obtained in this study may play a role in nitrate-dependent Fe(II)-oxidation, the reaction previously observed in rice paddy soils (21). More studies are necessary to determine if our Pseudogulbenkiania strains can oxidize metals in combination with nitrate reduction.…”

Do water regimes affect iron‐plaque formation and microbial communities in the rhizosphere of paddy rice?

“…On the other hand, the rhizosphere Fe(II) oxidation in paddy soils may be due to other biotic processes. For example, nitrate-reduction coupled to Fe(II) oxidation was reported in the rice paddy soil (Ratering and Schnell, 2001). The ubiquity and diversity of these anaerobic FeOB suggests that nitrate-dependent Fe(II) oxidation, light-independent reactions, have the potential to contribute to anoxic Fe oxidation in the rhizosphere, provided adequate concentrations of a suitable electron acceptor.…”